If we have a sound at 440Hz (A above middle C on a piano), that means that the sine wave of that sound is oscillating 440 times per second. There are some good answers here, but I was surprised that there were no visualizations of the actual waveforms yet, nor quantitative answers, which I found to be a very intuitive way of understanding what was going on. So clearly identical frequencies don't do you the favor to generally add without extinction occuring. PA systems actually have rather large problems for systems with multiple speakers to ensure there are no significant sound extinctions in the listening area: that's a science in itself. Higher frequencies, in contrast, have overlapping sound fields that you can tap into pretty well by moving your head. Multiple such instruments with that kind of almost pure sine tone quality are rather elusive to locate which is the reason that many speaker setups use only a single subwoofer. A wooden open organ pipe of "flute" type, in contrast, will be rather tricky with those notes, as will a bass ocarina. A bassoon will still be pretty locatable. You'll have no problems whatsoever locating a double bass playing a prolonged G1 or B1 (sort of the closest notes to mains hum depending on your locale). If you ever tried localising comparatively clean mains hum, you'll know that sine functions in isolation are a beast to deal with regarding hearing. Notes have disharmonicity, sine functions don't. Notes have overtones with characteristic phase relations, sine functions don't.
Notes have beginnings and endings (including attacks and decays), sine functions don't. You consider a note as a sine function with a certain frequency. What misconception do I have about sound? If we consider a note as a sine function with a certain frequency (ignoring timbre), So, in other words - yes, two waves of the same frequency CAN noticeably interfere with each other - but the stars need to be aligned JUST RIGHT for that to happen. And that's without taking into account all the echoes that normally happen in real life. But again - we're talking about a veeeery fine line here.
Now, there might be some locations where the effects might be more noticeable - for example on the line connecting the two centres. There will just be a few spots where they do, but these spots will move around and never stay in the same place. But because each wave started at a different location, these spots ALSO move around.Īnd that's your problem: unless both sounds emanate from precisely the same spot, their waves won't cancel out. And there are indeed places where the waves intersect and cancel each other out. Note that there are multiple sources of waves and the waves travel outward in circles. Because:Ĭheck out this picture of ripples in water that I picked up from Google:
Partial cancellation would be pretty noticeable too, and I think that even different timbres, if they were properly synced, would produce a noticeable distortion. Well, actually, they don't even need to be precisely 180° offset for you to notice effects. For this to happen, both waves need to arrive at your ear precisely in phase AND STAY THERE. I think the other answers underemphasize the number one factor why this doesn't happen in real life - phase. In a three dimensional acoustic space, if two notes are played in different locations, there's also a different summation that reaches each ear, which further reduces the chance of complete cancellation happening. They can still cancel somewhat, which gives you a comb filtering effect - this is exactly how phasing and flanging effects work. If you were to consider (say) a piano note as a sum of sinewaves, each of those sinewaves can be thought of as constantly changing in pitch and amplitude it's almost impossible for all of the sinusoidal components to be totally out of phase for any period of time, which is why you rarely hear two played notes cancelling each other out. I think the thing to take away is that a typical instrument sound is very much more complex than a sine wave. It sounds like you're observing that two notes played with a slight offset don't typically cancel each other out like this, and trying to reconcile that with the fact that two sine waves of constant amplitude could so easily cancel each other out. if your two waves are completely out of phase, then the result will indeed be silence. If your two sine waves are in phase, then you have a louder sinewave. If you play two sine waves of constant amplitude at the same frequency, then the result will be another sine wave.